feat(linkedin-studio): trend-store staleness nudge — SessionStart B-S3 [skip-docs]

SessionStart now warns (>=7d, warn-only) when the persistent trend store's
newest capture is stale, firing ONLY when the store already holds captures
(a never-scanned user is never nagged). Neutral wording — "scan for trends"
hits trend-spotter's own trigger; no hardcoded beat (de-niche invariant).

- store.ts: newestCaptureDate() — pure max-capturedAt staleness signal (SSOT)
- cli.ts: status [--json] subcommand (count + newest + daysStale)
- session-start.mjs: trendsNewestCapture() reads trends.json as raw JSON
  (no tsx spawn at session start) + the reminder line, beside import-staleness
- tests: +3 store tests (newestCaptureDate) + hook subprocess test (3 cases:
  >=7d fires, <7d silent, absent/empty silent + no crash)
- test-runner.sh: trends floor 21->24

Verified: trends 24/24 · all hook tests 131/131 · gate 89/0/0 · real render
confirms "Trend signals are N days old. Scan for trends…".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RigJBiRFNtFZKCz21qNbQ4

scripts/trends/src/store.ts

@@ -161,6 +161,17 @@ export function history(store: TrendStore, opts: HistoryOptions = {}): TrendReco
   return out;
 }
 
+/**
+ * The most recent capturedAt across the store (ISO date), or null when the
+ * store is empty. The staleness signal behind the SessionStart trend-freshness
+ * nudge (B-S3) and the `status` CLI subcommand: reuses history()'s newest-first
+ * ordering so the module has a single notion of "newest". Pure — the days-stale
+ * arithmetic (needs "today") lives in the caller, not here.
+ */
+export function newestCaptureDate(store: TrendStore): string | null {
+  return history(store, { limit: 1 })[0]?.capturedAt ?? null;
+}
+
 /**
  * Default store path under the per-user data dir (M0 data-path convention), so
  * the trend history survives plugin upgrades/reinstalls. `LINKEDIN_STUDIO_DATA`